- Add a default, move, and copy constructor for SsaEnv and use them
  when possible.
- Bring the Split method signature in line with Steal and
  Unimplemented.

Changes: 
Change-Id: I626142fbd1c1ed15c1852b9bfc8b39a0e936b839
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2204278
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67871}